If the editor is crashing I'd highly recommend checking your unpublished\cookedPC\environments (and others) folder for any epic upk's that may have been accidently modified and duplicated. If you find any delete them.

Regarding your map preview pic, attach the emissive function, rather than diffuse, to your texture sample to give it some light!

If ya still can't cook it, let me know and I'll recook and upload it for ya!

The stock upk packages will often get duplicated by doing certain things in the editor. One example is if you go into the properties of a static mesh and look at the default material for it and sync the browser with that material. Doing this for some reason appears to try and make a modification to the package hence creating a duplicate of it, without you knowing.